A typical photographic film of the type which is most commonly used in photography comprises a silver halide in crystal state appearing as a grain suspended in a gelatin emulsion. The emulsion usually comprises animal fats and protein supplying amino acids which participate in the photographic process in a manner believed to take place as follows. The silver halide crystals, for instance silver bromide, are removed from the emulsion during developing of the film unless the silver is converted to metallic form as a result of exposure to radiation. When exposed to visible light the silver bromide grain is bombarded with photons whose energy is sufficient to cause photoelectrons to be ejected from the valence band of the halide crystals with energy proportional to the energies of the arriving photons. These photoelectrons are ejected from the valence band of the silver halide into the conduction band and they migrate to a trapping region and combine with silver ions to form metallic silver. However, this combining is not irreversible, the metallic silver thus formed being only metastable. In the absence of the proper emulsion, these halides would tend to recombine with the silver, and the effect of the exposure to visible light would be lost. However, the emulsion tends to prevent such recombination. The halide ions are highly mobile and tend to migrate to the edge of the grain where they are absorbed by the gelatin of the emulsion. The emulsion digests the halogen in a way which is imperfectly understood, thereby effectively removing it from the grain and preventing it from destroying the latent image. The lattice site after photoejection is often referred to as a hole. These holes have a degree of mobility which is many orders of magnitude greater than the silver ions, and if they were not removed from the grain, these holes would recombine with electrons and destroy the image. The holes are stabilized by the emulsion and subsequently removed from the grain, thereby leaving the electrons with no place to go except to the silver ions where they form silver aggregate. If enough silver aggregate is formed at a trapping region, this region will become a developable image site which will remain developable in a latent state for some time.
There is a phenomenon known as the Herschel Effect which employs exposure to infra red light in the near region at wavelengths up to about 1.2 microns to selectively disperse latent image sites formed by prior exposure to visible light, thereby to record infra red events on silver halide-type films. According to this well known prior art process, a photographic film is first exposed to visible diffused light so that it is sensitized (fogged by light) uniformly across its surface, i.e., the sensitization being latent because the film is not yet developed. This means that in the grain, enough photoelectrons have been ejected to form upon the grain sites of silver aggregate which could be developed. According to the next step in the Herschel teachings the film is then exposed to near infra red radiation. In the areas of the film where no infra red is focused, the film retains its original fogged exposure, but in the areas where the infra red exposure is sufficiently concentrated this radiation impinges upon the grain sites and excites the aggregate to such an extent that the photoelectrons are dispersed from concentrated sites on the grain, whereby in these areas the original exposure to visible light to form a latent image is reversed, and the film when developed has lost the densifying effect of the original exposure to visible light. Thus, the image on the film when developed will show a gradation of density which varies inversely as the intensity of its exposure to the near infra red radiation. This Herschel Effect thus involves a subsequent reduction of photographic density (blackening) by subsequent exposure to near infra red, as is extensively discussed in the prior art, for example, in Patent 2,912,325 to Maurer, which cites literature also discussing the Herschel Effect.
It is also known to initially chemically desensitize a silver halide film, for instance for use in a photocopy machine, and then at the same time of its use to heat the film using infra red to re-sensitize it, and then exposing it while heated to an image to be reproduced. This type of process is recited in U.S. Pat. No. 3,250,618 to Stewart et al.